Polyurethane materials are made by reacting together a compound having more than one isocyanate function, i.e. a polyisocyanate, with a compound having more than one hydroxyl function, i.e. a polyol. In most cases a catalyst is added to the reaction mixture to accelerate the reaction and ensure complete and reproducible reaction conditions. A variety of catalysts is known and used for polyurethane manufacture, the most common being compounds of tin or mercury and also organic amine compounds. In many applications, metal catalysts are preferred because they are efficient and very effective. Whilst the use of heavy metal catalysts in polyurethane goods may not now be desirable, the alternative metals have disadvantages, particularly in shelf life and stability to hydrolysis. Titanium compounds, in particular, have the potential to offer economical alternatives which are of low toxicity compared with mercury for example. A problem with compounds of titanium and other metals such as aluminium, zirconium etc is that they are very effective catalysts but are rapidly hydrolysed in the presence of water to less catalytically active or inactive compounds.
Some polyurethane compounds, such as foams, are made from a reaction mixture to which a small percentage of water is added. Clearly in such a case the catalyst must be stable in the presence of water. In other cases the polyol composition of a two-part polyurethane reaction mixture contains water due to the hygroscopic nature of many polyols. It is common practice in the polyurethane supply chain to supply a two-part polyurethane formulation to an end-user in which the catalyst is already present, usually in the polyol-containing part. The user then mixes together the two parts and shapes the mixture before it cures to form a polyurethane material. The polyol, containing the catalyst, must therefore be stable during the period from manufacture to use and this may be a period of several months, depending on the application. If the catalyst/polyol mixture is not stable then changes in the catalyst activity can greatly affect the efficacy of the catalyst and thereby the properties of the cured polyurethane. It is therefore desirable to use as a catalyst a metal compound which is of relatively low toxicity compared with mercury or tin and which is stable in contact with a polyol over a long period of time, yet which is sufficiently active as a catalyst to be used in place of the currently used compounds.
It is an object of the invention to provide a compound which overcomes at least some of the problems with the catalysts of the prior art.